Eukaryotic Cells

Discovery of the Cell• Cells were first observed by the

English philosopher Robert Hooke.

• He observed a repeating pattern of pores in a thin slice of cork.– The word cells comes from the

Latin “Cella” meaning small room.

– No internal structures of the cell were visible to Hooke, so he did not consider them living.

• Nearly a century later, a Dutch scientist named Anton van Leeuwenhoek used a microscope to discover single-celled microorganisms in river water.– He followed this up

by observing living cells in blood, semen, and many other places.

Cell Theory• These discoveries led to the formation of the

Cell Theory:

1. Cells are the building blocks of all plants and animals.

2. Cells are the smallest functioning units of life.3. Cells are produced through the division of pre-

existing cells.4. Each cell maintains homeostasis.

The First Cells• The first life forms on Earth were likely single-celled

prokaryotic organisms.• Prokaryotic organisms are single-celled organisms

that do not have a nucleus.– Their DNA or RNA is usually

floating freely inside the cell.• Prokaryotic cells also do not

have any membrane bound organelles.

Eukaryotic Cells• Eukaryotes are organisms with much larger and more

complex cells than prokaryotes.– DNA is in a nucleus that is

bounded by a nuclear membrane.– Have membrane-bound

organelles• The largest eukaryotic cells

are 0.1mm to 1.0mm in size. – Why haven’t they evolved

any larger?

• The biggest limiting factor to cell size is surface area.– If the cell does not maintain a certain surface area-to-volume

ratio, it will not be able to transport nutrients and waste fast enough to survive.

• If the larger cell is instead broken down into 125 smaller cells, it will once again have enough surface area.

• This is why multicellular organisms exist!

The ostrich egg is one of the largest known single cells. Why is its size not limited by surface area?

• The eukaryotic cell can be divided into two major parts:– The nucleus is a separate

compartment that contains the DNA of the cell.

– The cytoplasm is the fluid portion of the cell outside the nucleus and any of the cells internal sub-structures,called organelles.

• The cytoplasm is separated from the external environment by a phospholipid bilayer called the plasma membrane.

Plasma Membrane• The plasma membrane of the cell provides these

functions:– Physical isolation between the internal cellular and

external environments.– Regulation of nutrient/waste exchange.– Sensing changes in the environment or receiving

communication signals from other parts of the body.– Connecting to other cells, giving body tissues a stable

structure.

• The main component of plasma membranes is phospholipids.– A class of lipid where one of the fatty acids is replaced by a

phosphate (PO4).• The two fatty

acid tails are hydrophobic; insoluble in water.

• The phosphate head is hydrophilic; water-soluble.

• The basic design of the cell membrane is a bilayer, a two-molecule thick film of phospholipids.– The phosphate heads form the

outer part that is in contact with the water.

– The fatty acids form the inner part that is away fromwater.

Membrane Proteins• Proteins embedded in the phospholipid bilayer

determine most of the membrane’s specific functions.

• Receptor proteins will respond to specific chemical signals.

• Carrier proteins transport substances across the cell membrane.

• Enzymes aid in chemical reactions.

• Anchoring proteins attach the plasma membrane to other structures.

• Recognition proteins identify the cell to the immune system as self or nonself; normal or abnormal.

Membrane Carbohydrates• In addition to proteins, chains of carbohydrates are

also found throughout the surface of the plasma membrane called glycoproteins.– These carbohydrates vary among species,

individuals, and even cell types in an individual.– Blood type (A, B, AB, O) is determined by carbohydrate

markers on your red blood cell membranes.

Permeability of the Lipid Bilayer

• The cell membrane has selective permeability, meaning some molecules pass through, others cannot.

Permeability of the Lipid Bilayer

• Nonpolar (hydrophobic) molecules can dissolve in the lipid bilayer and pass through the membrane rapidly– Examples: Oxygen, carbon

dioxide, hormones made of lipids

• Large polar (hydrophilic) molecules, cannot cross the membrane as easily.– Examples: Glucose, sucrose,

proteins

Passive Transport• Diffusion is the movement of molecules areas of

greater concentration to areas of lower concentration.– This is considered passive transport because no

energy is required.• The difference between the high and low

concentrations represents a concentration gradient.

• Osmosis is the diffusion of water through channel proteins embedded in the cell membrane.

• The direction of osmosis is determined by the concentration of solvent and solutes in a solution.– Solutes are solid substances dissolved in a solution,

like sugar or salts.– The solvent is the liquid component of the solution.

• Water follows the same rules of diffusion, moving from high to low concentration.– In other words, solutions high in solutes (salt,

sugar, etc) will draw in more water molecules.

• The Golden Rule of Osmosis:

Salt Sucks!

• Isotonic solutions, where the concentration of water molecules is the same inside and outside the cell, do not have a net movement of water.– Cells placed in isotonic

solutions are unaffected.

• Hypotonic solutions, where there is a greater concentration of water outside the cell, will cause a net movement of water into the cell.– This causes the cell to swell

and burst.

• Hypertonic solutions have a lower concentration of water than the interior of the cell, causing a net movement of water out of the cell.– The cell shrivels.

Reverse Osmosis Filtration• Reverse osmosis filters

work by moving water through a selective membrane against the normal concentration gradient.– This requires the

application of force (pressure).

– The resulting water has fewer dissolved solutes.

• In some forms of membrane transport, proteins aid the movement of molecules across the plasma membrane.

• If this process works with the concentration gradient, it is called facilitated diffusion.– This process is often used when the molecule is too pass

through the cell membrane on its own.

Active Transport• When carrier

proteins move substances againsttheir concentration gradient, ATP energy is required. – This is active

transport.

Vesicular (Bulk) Transport

• Cells can move large amounts of materials at the same time through membranous sacs called vesicles.

• If the material is imported into the cell, the process is called endocytosis.

• If material is exported out of the cell, the process is called exocytosis.

• Pinocytosis is a form of endocytosis also called “cell drinking,” because it occurs when the cell takes in large amounts of liquid.

• Phagocytosis, also called “cell eating,” occurs when the cell takes in large solid objects.

Eukaryotic Cell Anatomy

• The cytosol, or intracellular fluid, contains all the nutrients ions, and proteins needed by the cell.

• A eukaryotic cell has internal membranes that partition the cell into organelles (“little organs”).– Organelles are small structures within cells that have

specific jobs.

Animal Cell Anatomy

Smooth Endoplasmic Reticulum

CytosolVacuole

Centriole

Villi

Mitochondria

Nuclear Membrane

Golgi ApparatusNucleolusChromatin (DNA)

Nuclear Pores

Rough Endoplasmic ReticulumPlasma Membrane

Lysosome

The Cytoskeleton• The cytoskeleton is a framework

of protein filaments that provides structure and support to the cell.

• The cytoskeleton of some cells forms finger-shaped projections called microvilli that increase surface area.– Needed by cells that absorb a lot of

material (intestines, kidneys, etc)

• A centriole is a cylindrical structure made of cytoskeleton fibers.– Direct the movement of chromosomes during cell

division.

• Cilia are long, slender extensions of the plasma membrane.– Able to independently

move using energy from ATP.

• Flagella are similar to cilia, but much longer.– Sperm cells are the only human cells with flagella.

Protein Metabolism• Proteins in cells are removed and recycled by

proteasomes.– Usually damaged, denatured, or infectious proteins.

• All of the proteins used by cells are produced by ribosomes.– Some are freely floating throughout the cytoplasm, others

are attached to the endoplasmic reticulum.

• The Endoplasmic Reticulum is a network of membranes directly connected to the membrane of the nucleus.

• The smooth ER does not contain any attached ribosomes.– Produces lipids and carbohydrates (such as glycogen) for the

cell.

• The rough ER has ribosomes permanently attached to its membranes.– The ribosomes produce proteins.– The ER can package these proteins and transport them

to the Golgi.

Golgi Apparatus• The Golgi Apparatus is made of several flattened

discs that can produce three different types of vesicles:– Lysosomes contain digestive enzymes for breaking down

and recycling cell contents.– Secretory vesicles contain hormones or enzymes to be

released outside the cell.– Membrane renewal vesicles contain lipids and proteins to

be added to the cell membrane.• A typical path of a protein formed by a cell goes:

– Rough ER → Golgi → Cell membrane → Released by cell

• Lysosomes are sometimes referred to as “suicide sacs” because they release their enzymes into the cytosol when the cell is damaged and kill it.

Perioxisomes• Similar to lysosomes, perioxisomes carry enzymes.

– Break down fatty acids.• Perioxisomes produce hydrogen perioxide (H2O2) as a

waste product.– H2O2 is a free radical, meaning it has unpaired electrons that

can react with and damage other molecules.– Also contains an enzyme to break down the H2O2.

H2O2 ® 2H2O + O2

Mitochondria

• All of the cell’s energy demands are met by mitochondria.– As glucose is broken down

into carbon dioxide and water, the energy released is captured by converting ADP into ATP.

• This process is called aerobic respiration, as it consumes oxygen.

• Heart muscle cells, which have high energy demands, are about 30% mitochondria by volume.

• Red blood cells have no mitochondria at all.

Nucleus• The nucleus is separated from the rest of the

cell by a double-membrane nuclear envelope.• Communication between

the nucleus and cytosol occurs through openings called nuclear pores.

• The nucleolus is a small structure that assembles ribosomes.

• All of the cell’s instructions are contained within molecules of DNA inside the nucleus.

• The DNA can take two forms:– Chromosomes, when it is

condensed and tightly coiled.

– Chromatin, when it is loosely coiled.

Onion root tip cells, 160x.Image from www.microscopy-uk.org

• In the same way that the boss gives the orders to control a factory, the nucleus controls the cell by directing the synthesis of proteins.

Plant Cells• Plant cells have many of the same structures as animal

cells, with a few differences.

Cell Wall

Cell WallPlasmodesmata

Plasmodesmata

Cell Membrane

Cell Membrane

Nucleolus

NucleusNuclear

Envelope

Central Vacuole

Central Vacuole

Mitochondria

ChloroplastRough ER

Ribosomes

Smooth ER

Smooth ER

Golgi

Cytosol

Plant Cell Anatomy

Differences in Plant Cells• Plant cells have many of the same structures as animal

cells, with a few important differences:• Plant cells have chloroplasts, which contain the green

pigment chlorophyll needed to perform photosynthesis.

• A rigid cell wall made of cellulose covers the cell membrane.– This provides the plant with structure and protection,

especially important for its stems.– The cell wall has tiny holes called plasmodesmata so that

membrane transport can still occur.

• Due to their cell walls, plant cells do not explode when placed in hypotonic solutions. Rather, they become firm and turgid. This is normal.– In isotonic solutions, they will start to become limp and wilt. – In hypertonic solutions, they will shrivel inside the cell

walls. This is called plasmolysis.

• This is why grocery stores regularly mist their produce with water.